-LRB- CNN -RRB- The HBO documentary `` Kurt Cobain : Montage of Heck '' is already drawing rave reviews on the film festival circuit , including sold-out showings at both Sundance and the True/False fests before its television premiere on May 4th .

However , anticipation for the film 's soundtrack is about to hit fever pitch as director Brett Morgen revealed that the accompanying album will feature `` a mind-blowing 12-minute acoustic Cobain unheard track , '' the filmmaker tweeted -LRB- via Loudwire -RRB- .

Kurt Cobain documentary set to premiere at Sundance

Morgen did n't share any other details regarding the song other than it will feature on the `` Montage of Heck '' soundtrack . While an April 7th release date has been scheduled for the film 's companion book , which features `` a mixture of animation stills , rare photography and other treasures from Kurt Cobain 's personal archive , '' no date or further information has been announced regarding the film 's soundtrack .

Kurt Cobain doc ` Montage of Heck ' adds rarities-filled companion book

Rolling Stone talked to Morgen at the Sundance Film Festival , and the filmmaker revealed that the documentary 's score `` is all unreleased Cobain music . ''

`` They do n't have titles . Before people saw the movie , there were these weird press releases focusing on the unreleased music . And it 's like : It 's a movie . We 're not going to stop it and play a song for four minutes , '' Morgen said . `` But nobody in Kurt 's life -- not his management , wife , bandmates -- had ever heard his Beatles thing -LSB- a snippet of ` And I Love Her ' -RSB- . I found it on a random tape . It 's a Paul -LSB- McCartney -RSB- song . How 's that for shattering the myth ? ''

Sundance 2015 : Intimate ` Kurt Cobain : Montage of Heck ' doc stuns at premiere

Morgen added that he hoped to put out one of the Cobain 's personal cassettes - `` Tape 59 : Montage of Heck '' - as a special release on `` Independent Record Store day , like unannounced , but it did n't happen . ''

See the original story at RollingStone.com .

Copyright © 2015 Rolling Stone .

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The album will feature a 12-minute acoustic Cobain unheard track

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The doc is already winning rave reviews

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Filmmaker wants to release one of the Cobain 's personal cassettes